Anchor line tensioning method

ABSTRACT

Method for mooring an offshore structure includes: 
     providing a mooring line having one end connected to the sea bed and a connector at a free end;
 
lifting the free end onto a deck of an anchor line handling vessel;
 
connecting on the anchor line handling vessel a chain tensioner with a connection side to the connector;
 
providing a working chain having a short tensioning part and a long installation part into the entry side of the chain tensioner;
 
holding a free end of the installation part while lowering the chain tensioner below water level;
 
picking up a free end of a pendant attached to the offshore structure, and attaching it to the free end of the installation part in an interconnect position on the anchor line handling vessel;
 
placing the interconnect position overboard;
 
connecting a lifting cable on the anchor line handling vessel to the tensioning part below water level; and
 
exerting an upward tensioning force on the tensioning chain part.

FIELD OF THE INVENTION

The invention relates to an anchor line tensioning method for mooring afloating offshore structure, such as a floating production storage andoffloading system (FPSO), a semi-submersible platform, aloading/offloading buoy or any other floating structure, to the sea bed.The invention also relates to a floating offshore structure having amooring line configuration for carrying out the anchor line installationmethod.

BACKGROUND OF THE INVENTION

From WO 03/013950 in the name of the applicant, an anchor lineinstallation method is known according to which a pre-installed anchorline carrying a connector at its free end, is retrieved by theinstallation vessel. The connector may be formed by a chain stopper sothat it functions as a chain tensioner device. A pulling line attachedto a winch on the installation vessel is guided along a sheave on thechain tensioner device on board of the installation vessel. Theinstallation vessel also picks up a free pendant end that is connectedto the floating structure and the pulling line is connected to this freependant end. The chain tensioner device is over boarded and is connectedbelow water level with the pendant by hauling in the pulling wire on thewinch.

By placing the chain tensioner device externally from the mooredstructure, in line with the anchor leg, the installation andcommissioning of the tensioning system is removed from the constructionyard schedule of the moored structure. Also, a reduction in chain tablesize and removal of installation aids from the turret is achieved incase the chain tensioner is used in turret-moored offshore structures.For spread-moored FPSO's, utilising the known external in-line tensionerresults in space saving at the topside due to the absence of tensionerdevices such as chain jacks and chain stoppers and the absence ofstorage space for excess installation chain. Furthermore, no maintenanceis required on chain jacks on the moored offshore structure and duringre-tensioning operations no disruption of the production and processingoperations occurs.

The known method has as a disadvantage that the pendant that isconnected to the floating structure has a relatively long length, whichmakes it difficult to store on and to deploy it from the floatingstructure. Furthermore, checking for a proper connection between theknown chain tensioner device and the pendant requires underwaterinspection, such as by divers. If for some reason a proper connectionbetween the pendant and the tensioner device is not made, the connectionoperation needs to be interrupted.

From U.S. Pat. No. 7,059,262 a mooring line connection method is knownwherein a pre-installed mooring line with at its free end a clampingdevice is placed on the sea-bed. The clamping device comprises a slottedsheave or pulley and a chain stopper that is attached to a recoverypendant and a submersed buoy. The buoy is attached to a pull in wire onan anchor-handling vessel via a ROV and the clamping device is hoistedon board of the handling vessel. On board of the vessel the pendant fromthe floating structure, having at its free end a chain part, is pulledthrough the clamping device, and is attached to the pulling wire. Next,the clamping device is over boarded and the mooring line is pulledupward such that the pendant chain part moves along the chain stopper ofthe clamping device that functions as a ratchet allowing only onedirectional movement of the chain, until the right tension in themooring line is achieved.

In the known method, the connection between the chain tensioner and thechain part of the pendant connected to the floating structure is madebelow water level, which results in difficult control over theconnection procedure. Also abandoning the clamping device on the seabedmay cause ingress of sand or small stones into the clamping deviceresulting in malfunctioning or blocking of the tensioner device. Again,the relatively long length of the pendant connected to the floatingstructure in this mooring line installation method results in relativelydifficult handling.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an anchorline installation method, which allows controlled and easily monitoredconnection of the mooring line that is attached to the seabed, to thependant connected to a floating structure.

It is a further object of the present invention to provide an anchorline installation method, which allows easy handling of the pendantsegment connected to the floating structure.

It is again an object of the invention to provide an anchor lineinstallation method in which the risk of malfunctioning of the chaintensioner device is mitigated.

It is another object of the present invention to provide an anchor linetensioning method allowing controlled tensioning of the anchor linesusing anchor handling vessels having relatively low-powered engines.

Hereto an anchor line installation method according to the presentinvention comprises the steps of:

-   -   Providing a mooring line, with one end connected to the sea bed        and comprising at a free end a connector member,    -   Lifting the free end onto a deck of an anchor line handling        vessel,    -   Connecting on the anchor line handling vessel a chain tensioner        with a connection side to the connector member,    -   Providing a working chain into the entry side of the chain        tensioner, the working chain having a relatively short        tensioning chain part and a relatively long installation chain        part,    -   Holding a free end of the installation chain part connected to        the anchor line handling vessel while lowering the chain        tensioner below water level,    -   Picking up a free end of a pendant attached on one end to an        offshore structure that is to be moored, and attaching the free        end of the pendant to the free end of the installation chain        part in an interconnect position on the anchor line handling        vessel,    -   placing the interconnect position over board from the anchor        line handling vessel,    -   Connecting a lifting cable of a lifting device on the anchor        line handling vessel to tensioning chain part below water level,        and    -   Exerting an upward tensioning force on the tensioning chain part        via the lifting cable.

The chain tensioner is fixedly attached to the mooring line with itsconnecting end on board of the anchor line handling vessel, and does notcome into contact with the sea bed such that ingress of sand or stonesis avoided.

The tensioning chain part is passed through the chain tensioner on boardof the anchor line handling vessel, which allows for a controlledoperation with good possibilities of diverless visual inspection.

The use of the tensioning chain part that is attached with its upper endto the pendant connected to the floating structure, allows that thependant can be a relatively short line section connected to the floatingstructure, such that handling thereof is facilitated. The anchor legconnected to the chain tensioner can be for the largest part made ofsynthetic rope material, such as polyester of a length of 1000 m ormore, for instance suitable for deep water of a depth of severalthousands of meters. Using the method of the invention, re-tensioning ofsuch synthetic anchor lines now is possible to compensate for creepelongation over time by pulling in a new installation chain part.

In an embodiment, the installation chain part of the working chain is atleast 0.5 times the length of the pendant, preferably at least 1 timesthe length of the pendant, most preferably at least 1.5 times the lengthof the pendant. The pendant can be formed of a chain, steel or polyesterwire rope or combinations thereof. The free ends of the pendant andinstallation chain part can be interconnected via releasable bowshackles, chain links or any other suitable connector devices. It is forinstance within the scope of the invention that the installation chainpart comprises at its free end a cable with an eye for connecting to aneye on a cable at the free end of the pendant.

In a further embodiment of a method according to the invention, theanchor line handling vessel is connected to the offshore structure in aholding point via a holding line that is connected to a pulling deviceon the anchor line handling vessel, wherein an upward tensioning forceis exerted on the tensioning chain part by the pulling device exerting apulling force on the anchor line handling vessel in the direction of theoffshore structure via the holding line.

By horizontally pulling the anchor line installation vessel, forinstance via its stern, in the direction of the moored structure, itdoes not have to use the full power of its thrusters to generate ahorizontal force upon tensioning the mooring line via the pullingdevice. In this manner, a larger selection of anchor line handlingvessels having smaller sized engines can be used. Furthermore, anchoringthe handling vessel against the offshore structure in a fixed horizontalposition provides a stable and safe tensioning configuration in whichthe tensioning operation can be properly controlled.

In a further embodiment of a method according to the invention, thelength of the holding line is shorter than a horizontal distance of thetensioner to the holding point such that the tensioning chain partextends substantially in line with the mooring line. Preferably theholding line keeps the installation vessel in a such a horizontalposition that the chain tensioning part and the pulling line extendwithin an angle of at most +/−30 degrees around the operationaldirection of the upper part of the anchor leg, preferably in line withit, such that during tensioning forces are exerted on the anchor in adirection corresponding to the in-use forces. Hereby it is achieved thatthe anchor is pulled in line with the direction of regular use, and canin that direction provide an optimal anchoring force as many anchors,such as drag anchors, fluke anchors or suction piles, are relativelysensitive to the direction of the tension load.

Preferably the holding point on the moored structure is situated abovewater level and the holding line extends substantially in a horizontaldirection to the installation vessel. In a preferred embodiment of amoored offshore structure according to the invention, the mooring pointis situated in a lower half of the moored structure, for instance on asubmerged turret moored buoy, a chain table of the turret or on the hullof the structure. In case of connection to the lower part of the hull,for instance via a uni-joint, the anchor line connection is subject toreduced fatigue compared to connection to a known gypsy wheel and chainjack tensioner device on deck of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of an anchor line tensioning method according to theinvention will by way of non-limiting example be described in detailwith reference to the accompanying drawings. In the drawings:

FIG. 1 shows a side view of an FPSO anchored to the sea bed utilising amooring leg configuration according to the present invention,

FIG. 2 shows a the first stage of an anchor line tensioning methodaccording to the invention,

FIGS. 3a and 3b show a side view and a top view of a chain tensioner foruse in the tensioning method of the invention,

FIGS. 4-6 show side views of further stages of bringing together thependant chain and the installation chain part on the anchor linehandling vessel,

FIG. 7 shows a top plan view of the stern part of the anchor linehandling vessel, the pendant chain and the installation chain part beingattached to respective winches on the vessel,

FIG. 8 shows side view of the anchor line handling vessel in theposition of connection of the installation chain part to the pendantchain,

FIGS. 9 and 10 show top plan views of the stern part of the anchor linehandling vessel upon interconnecting the installation chain part and thependant chain,

FIG. 11 shows a side view of the hook up of the tensioning chain partand a holding hawser to the anchor line handling vessel,

FIG. 12 shows a side view of the tensioning of the anchor line,

FIGS. 13 and 14 show side views of the disconnecting step of holdinghawser and tensioning chain part from the anchor line handling vessel,and

FIG. 15 shows an anchoring point on the hull of a moored offshorestructure in the form of a uni-joint.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a moored offshore structure 1 according to the invention inthe form of a spread-moored FPSO. The FPSO carries a number ofhydrocarbon production risers 2 attached with their lower ends to a subsea hydrocarbon well and with their upper ends supported near a deck 3and in fluid connection with storage and/or processing equipment onboard of the FPSO 1. The FPSO is anchored in a lower half H_(l), of itsheight H between keel level 4 and the level of deck 3. In the lower halfH_(l), the FPSO is provided with anchoring points 5,6 on its hull. Inthe anchoring points 5,6 the FPSO is connected via uni-joints to groupsof anchor legs 7,8 which with their lower ends are anchored to the seabed 10 via anchors or suction piles. Although the embodiment in FIG. 1shows a spread-moored FPSO, the invention is also applicable to othermooring configurations, such as for instance turret-mooredconstructions, structures moored via a disconnectable submerged buoy andto other offshore structures such as Floating Storage and RegasificationUnits (FSRU's) for processing of liquefied natural gas, semi-submersiblestructures, single point mooring buoys and the like.

FIG. 2 shows a side view of a first stage of the anchor leg connectionand tensioning procedure according to the invention. In FIG. 2, theanchor leg 11 may be installed, tensioned and be picked up by the anchorline handling vessel 12 or may be have been pre-installed by anothervessel and connected to the seabed 10 and tensioned in the prior step.The anchor line handling vessel 12 picks up the top part 13 of theanchor leg 11, which top part may comprise a polyester rope section 14and a chain section 15, for instance of a length of 20 m. The chainsection 15 is clamped in a clamping device 35 (see FIGS. 7, 9 and 10) atthe stern 16 on deck of the vessel 12.

As shown in FIGS. 3a, 3b , on deck of the vessel 12, a chain tensioner17 is fixedly attached with a connection side 18 to the upper end of thechain section 15 via a chain link 14 and connector member, such as a bowshackle 22. It is also envisaged that instead of chain section 15, awire rope or other line section is utilised to connect to side 18 of thechain tensioner device 17 using a suitable connector member. Duringconnection of the upper end of chain section 15 to the chain tensionerdevice 17, the chain section 15 remains clamped on deck of the vessel12. A working chain 20, having a length of for instance 200 m, is ondeck of the vessel 12 movably passed through the chain tensioner 17, byfeeding tensioning chain section 28 that is formed by the lower part ofthe working chain 20 via an entry side 19 of the chain tensioner througha chain stopper 25 and along a sheave 23. The chain stopper 25 functionsas a ratchet and only allows passage of the working chain 20 through thetensioner housing 27 in the direction of the shorter tensioning chainpart 28, that becomes longer upon pulling it in the direction indicatedas T. The tensioning chain part 28 of the tensioning chain 20 is guidedvia the rotatable sheave 23 out through an opening 26 (see FIG. 3b ) inthe housing 27 of the chain tensioner 17. A pulling eye 24 of relativelylarge diameter is attached to tensioner chain part 28 for handling by aremote operated vehicle (ROV). Next, as can be seen in FIG. 4, theworking chain 20 is attached to a work wire 30 on the anchorline-handling vessel 12, the work wire 30 being wound on the drum of amain winch 31, and the chain tensioner 17 is over boarded. The workingchain 20 is paid out via the work wire 30 via the main winch 31, whilethe vessel 12 moves rearwards towards the FPSO 1.

From the FPSO 1, a pre-installed pendant chain, or chain tail 32 isrecovered and is attached to a tugger winch 33 on the vessel 12 via atugger winch wire 34 (see FIG. 7). As can be seen in FIG. 6 and FIG. 7,the pendant chain 32 is paid out from the FPSO 1 and is recovered on thestern 16 of the vessel 12 where it is fixed in the clamping device 35,between movable jaws 39,40. Next, the anchor line-handling vessel 12 ismoved away from the FPSO 1 to recover the free end 45 of the workingchain 20 to the deck of the vessel 12. The tugger winch wire 34 may bepaid out from winch 33 as required.

As shown in FIGS. 8 and 9, the vessel 12 moves away from the FPSO 1 alittle under 200 m such that the depth of the pendant chain 32 belowwater level 46 is about 40 m. The tension in the pendant chain may bearound 40 T while the stern thrust of the anchor-line handling vessel isabout 15 T. The fixed end of pendant chain 32 is attached to theanchoring point 47 on the hull of the FPSO 1 via a universal joint 49.The free end 45 of the working chain 20 and the free end 50 of thependant chain 32 are both held by the clamping device 35 at a closemutual distance, as can be seen in FIGS. 9 and 10, such that they can beeasily interconnected on the deck by a connector member 51 which maycomprise for instance two interconnected bow shackles acting on the openchain links at the end of chains 32 and 20.

Next, as can be seen in FIG. 11 the clamping device 35 is opened and thejaws 39,40; 43, 42 are simultaneously released such that interconnectedchains 20, 32 are over boarded. At the interconnect position 52 of thechains 32, 20, on the end of the pendant chain 32, the connector member51 is situated. The anchor line-handling vessel 12 is moved with itsstern towards the FPSO to recover a holding hawser 57, attached to theFPSO 1 in a holding point 53 on the hull of the FPSO 1, situated nearanchoring point 47. The free end of the hawser 57 is connected to thework wire 30 and main which 31. Next, a remotely operated vehicle (ROV)56, connected to the vessel 12 via an umbilical 55, is over boarded andattaches tugger winch wire 34 to the pulling eye 24 of tensioning chainpart 28.

Then, as shown in FIG. 12, the tugger winch 33 is operated to pull inthe tensioning chain part 28 to the required length or tension, e.g. 118m and 30 T. In the present case, the anchor line 11 is of a relativelylarge length, such as several km, and the length of the tensioning chainpart 28 in this case is relatively long to compensate for elongation ofthe anchor line. In case the anchor line 11 is shorter, for use inshallower waters, the length of the chain part 28 will be shorter.

Thereafter, the main winch 31 is operated to pull in the holding hawser57 to tension the system. In FIG. 12, the depth of the chain tensioner17 below water level 46 may be about 100 m, the length of the hawser 57being about 60 m at a tension of 90 T, the upward tension in tensioningchain part 28 being 150 T at a length of about 75 m, the tension in theanchor leg 11 being about 260 T, the length of the pendant chain 32 andinstallation chain part 21 being about 120 m. The ROV 56 monitors thepassage of the tensioning chain part 28 through the chain tensioner 17until a reference chain link (having a predetermined distinctivecharacteristic, such as colour) reaches a preset position on the chaintensioner 17.

As can be seen in FIG. 13, the tensioning chain part 28 is thereafterlowered and detached, while the hawser 57 is returned to the FPSO 1,while remaining attached to the holding point 53, to complete theprocedure.

FIG. 15 shows the hull of the FPSO 1 having in anchoring point 47 abracket attached to the outer hull carrying a flange 60. A 4 m longmoment arm 63 is connected to the flange to form a universal joint,rotatable around axis 61 and around an axis extending perpendicular tothe plane of the drawing in hinge point 62. The pendant 32 is fixed tothe end of arm 63. In this case, the anchoring point is situated belowwater level

1. A chain tensioner, comprising: an elongate tensioner housing; aconnection side with a connector member and an entry side; wherein theconnection member is configured for connecting to a mooring chainsection, and wherein the entry side is configured for feeding a workingchain to a tensioner housing, a sheave; and an opening in the tensionerhousing for passage of a tensioning chain section of the working chainfrom the entry side through the tensioner housing along the sheave,wherein the connection member and the entry side are positioned on aline along a longitudinal axis of the tensioner housing, the sheave ispositioned on the tensioner housing in between the connection member andthe entry side, and is aligned with the connection member and the entryside along said longitudinal axis, and wherein a center of rotation ofthe sheave is positioned at a perpendicular distance from said line,with the perpendicular distance substantially equal to a radius of thesheave.
 2. The chain tensioner according to claim 1, wherein said lineis a tangent of the sheave circumference.
 3. The chain tensioneraccording to claim 1, wherein the chain tensioner further comprises achain stopper.
 4. The chain tensioner according to claim 1 incombination with the the mooring chain section and the working chain,wherein the connection member is connected to the mooring chain section,and wherein the tensioning chain section of the working chain passesthrough the opening in the tensioner housing from the entry side throughthe tensioner housing along the sheave.
 5. The chain tensioner accordingto claim 4, wherein said line is a tangent of the sheave circumference.6. The chain tensioner according to claim 4, wherein the chain tensionerfurther comprises a chain stopper.
 7. The chain tensioner according toclaim 5, wherein the chain tensioner further comprises a chain stopper.